Kotlin List associateWithTo()
Syntax & Examples

Syntax of List.associateWithTo()

The syntax of List.associateWithTo() extension function is:

fun <K, V, M : MutableMap<in K, in V>> Iterable<K>.associateWithTo( destination: M, valueSelector: (K) -> V ): M

This associateWithTo() extension function of List populates and returns the destination mutable map with key-value pairs for each element of the given collection, where key is the element itself and value is provided by the valueSelector function applied to that key.

✐ Examples

1 Example

In this example,

We create a list named list1 containing the integers 1, 2, 3, 4.
We create an empty mutable map named map1.
We use the associateWithTo function on list1 to associate each element with its square in map1.
The resulting map map1 contains key-value pairs where each key is an element of list1 and the value is its square.
We print map1 to standard output using println.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4)
    val map1 = mutableMapOf<Int, Int>()
    list1.associateWithTo(map1) { it * it }
    println(map1)
}

Output

{1=1, 2=4, 3=9, 4=16}

2 Example

In this example,

We create a list named list2 containing strings "apple", "banana", "cherry".
We create an empty mutable map named map2.
We use the associateWithTo function on list2 to associate each string with its length in map2.
The resulting map map2 contains key-value pairs where each key is a string from list2 and the value is its length.
We print map2 to standard output using println.

Kotlin Program

fun main(args: Array<String>) {
    val list2 = listOf("apple", "banana", "cherry")
    val map2 = mutableMapOf<String, Int>()
    list2.associateWithTo(map2) { it.length }
    println(map2)
}

Output

{apple=5, banana=6, cherry=6}

3 Example

In this example,

We create a list named list3 containing doubles 5.5, 10.0, 15.5.
We create an empty mutable map named map3.
We use the associateWithTo function on list3 to associate each double with its integer part in map3.
The resulting map map3 contains key-value pairs where each key is a double from list3 and the value is its integer part.
We print map3 to standard output using println.

Kotlin Program

fun main(args: Array<String>) {
    val list3 = listOf(5.5, 10.0, 15.5)
    val map3 = mutableMapOf<Double, Int>()
    list3.associateWithTo(map3) { it.toInt() }
    println(map3)
}

Output

{5.5=5, 10.0=10, 15.5=15}

Summary

In this Kotlin tutorial, we learned about associateWithTo() extension function of List: the syntax and few working examples with output and detailed explanation for each example.